1. Field
The present invention relates to photogrammetry, and, more particularly, to systems and methods consistent with arrays of image capturing devices directed to the acquisition of images regarding large area objects or large areas.
2. Description of Related Information
Aerial and satellite imagery of the earth is used for a wide range of military, commercial and consumer applications. Recent innovations sometime include components that process and compress large amounts of images or serve entire photo imagery maps on the Internet, and advances such as these have further increased the demand for imagery. However, existing systems often involve overly complex components, require high capital expenditures, and/or have high operating costs, among other drawbacks. They are unable to yield imagery within narrower timeframes and operating regimes, or otherwise provide the higher resolution, presently desired.
In general, existing photogrammetry imagery solutions are failing to meet the increasing demand for more timely and higher resolution imagery. According to principles consistent with certain aspects related to the innovations herein, camera systems used for aerial photogrammetry must address two conflicting requirements.
First, it is vital that the camera system's lens and focal system parameters (known as Interior orientation), as well as its position in space and look angle (known as Exterior orientation) are precisely calculated. A photogrammetric solution known as bundle adjustment may be used to calculate Interior and Exterior orientation information for the camera and for each photo taken by the camera. Such calculations often represent a pre-requirement for enabling merging of individual photos into seamless photomaps. One way of achieving the required level of accuracy is to take multiple photos, with a large amount of redundant overlap between photos. Common features visible in multiple photos can then be identified and used to calculate camera interior and exterior parameters.
Second, it is desirable that aerial surveys be completed quickly. This provides several advantages like reduced operating costs and minimized delays stemming from unfavorable environmental or surveying conditions such as inclement weather. An effective way to increasing the amount of ground area captured, measured in km2 per hour, is to minimize the amount of redundancy between photos.
As such, the need to increase redundancy between photos to enable accurate photogrammetric positioning of the photos must be balanced with the need to decrease redundancy between photos to complete surveys at a lower cost.
One existing approach uses “push-broom” linear detector arrays to minimize redundant capture and maximize capture rate. This approach minimizes the amount of redundancy and so increases capture rate. However, one drawback of this approach is that it sacrifices positional accuracy calculated from redundancy in the photos themselves, and so other complex methods must be used to accurately calculate camera system position information.
Another existing approach is to increase the size of the camera system being used, i.e., in terms of the megapixel count for the cameras or camera arrays. Here, for example, multiple sensors and/or lenses may be combined in a single unit to maximize the megapixel count for the camera system. While this approach may increase the megapixel size of the camera system, it fails to address reduction of redundancy between photos.
Various systems are directed to minimizing amounts of redundant overlap between photos in a survey. Some existing camera systems, for example, are mounted in a fully gyroscopically stabilized platform which in turn is mounted in an aircraft. These systems may insulate the camera(s) from excessive yaw, pitch and/or roll, and enable a lesser amount of redundancy to be used between photos. However, such stabilization systems are expensive and heavy, and suffer from drawbacks like higher camera system costs and the need for larger aircraft to fly the survey.
Other existing systems adapted to estimating camera pose and reducing redundant photo overlap requirements sometimes include one or more IMU (Inertial Measurement Unit) systems with the camera system to provide measurement of the camera's yaw, pitch and roll. Such IMU systems, however, are complex and expensive, and the ability to utilize units of sufficient accuracy is often constrained by export restrictions that prohibit their use in many countries.
Certain other existing systems may include D-GPS (Differential GPS) units that enable estimation of the camera systems position when each photo is taken. These units, with appropriate post-survey (i.e., post-flight) processing, allow position to be estimated to centimeter accuracy. However, D-GPS units are expensive, and typically require a direct signal path to the GPS satellites in order to measure the signal phase later used to calculate precise position. Thus drawbacks of these systems include the requirement that aircraft must take very gradual/flat turns at the end of each flight line in a survey, to ensure that portions of the aircraft such as a wing do not block the D-GPS antennae's view of the satellites. These gradual/flat turns add significantly to the amount of time required to fly a survey.
Still other existing systems provide improved photogrammetric solution accuracy via use of industrial grade high quality lenses, which can minimize the amount of Interior orientation error induced by lens distortions. However, such high quality lenses add significantly to the cost of the camera system.
Even with such techniques, aerial surveys still require a significant amount of overlap between photos in order to ensure production of high quality photomaps. The amount of overlap between photos varies depending on the application and desired quality. A common overlap is 30/80, meaning 30% side overlap with photos in adjacent parallel flight lines, and 80% forward overlap with photos along a flight line. This amount of overlap allows a feature to be identified on average in about 5 photos, which, in combination with the stability and position techniques discussed above, is sufficient to enable accurate photogrammetric bundle adjustment of photos.
However, side overlap of 30% and forward overlap of 80% means that only 14% of each photo covers new ground. About 86% of the photo information taken is redundant in terms of the final photomap product produces, so aerial surveys are fairly inefficient in terms of the amount of flying required to cover an area. Also, the redundant photo data must be stored and later processed, which further increases costs.
While greater levels of redundancy, or overlap, increase the ability to precisely calculate Exterior and Interior orientation for the camera system, such redundancy is largely wasted when creating a final photomap. This is because significantly more redundant imagery is captured than needed to create a photomap, which also increases the time and cost required to fly a survey. A satisfactory balance between these considerations is not available in a variety of other known systems, which all suffer from additional shortcomings.
For example, many existing systems for aerial photography require very expensive camera solutions that are typically purpose-built for the application. Such systems suffer the drawback that they cannot use COTS (Commercial Off the Shelf) cameras/hardware. Further, the heavy weight and high cost of these camera systems often requires the use of twin-engine turbo-prop aircraft, which further drives up operating costs since these aircraft are much more expensive to operate than common single engine commercial aircraft like the Cessna 210. Moreover, specific mounting requirements for such camera systems frequently require custom modification of the aircraft in order to mount the camera system.
Further, conventional large format aerial survey cameras are typically large, heavy and expensive. It is often impossible to configure systems of such cameras to take oblique photos at the same time as taking nadir photos. Oblique photography is very widely used in intelligence gathering and military applications, and has recently become popular for consumer applications. Oblique photomaps provide a view of objects such as houses from the side, where as nadir, or overhead, photomaps look from directly overhead and don't show the sides of objects. Oblique photography is also desirable to enable textures to be placed over 3D object models to increase realism. Existing systems that do provide oblique imagery often suffer additional limitations. For example, capture rates can be very low, and the aircraft typically must fly at low altitudes in order to capture high resolution oblique images. Moreover, minimal overlap is generally provided between photos from different obliques, making it difficult or impossible to create photogrammetrically accurate photomaps.
Furthermore, many existing systems have limited resolution (megapixels) per image and use much of their available resolution to capture redundant data used to accurately calculate camera position and pose. These systems suffer drawbacks when identification of smaller objects from the images is desired, such as the requirement that they fly surveys closer to the ground to capture images of high enough resolution to identify such objects. For example, a camera system must survey (fly) at 3,000 feet altitude to capture 7.5 cm pixel resolution photos using a Vexcel UltraCam-D camera. Flying at such a low altitude causes multiple problems. First, turbulence and thermals are much worse at these lower altitudes, which makes the flying rougher and more difficult for the pilot, and decreases the stability of the camera system. Secondly, flights over urban areas are more difficult at these altitudes, as ATC (Air Traffic Control) has to juggle the flight paths for the survey aircraft—which needs to fly a consistent set of flight lines—with incoming and outgoing flights from airports surrounding the urban area. Interruptions in survey flights at these altitudes cause significant delays in capturing the survey, further increasing costs.
Many existing systems also require large amounts of data storage onboard the platform or aircraft. These systems typically include local image capturing systems and/or storage devices, to which image data is transmitted or downloaded from the cameras. Often, the storage must be both fast enough to store photo data streaming from the cameras, and capable of storing enough data to enable a reasonable amount of flying time. Further, many such systems use RAID based hard disk storage systems to store in-flight captured data. However, hard disks are sensitive to low air pressure at higher altitudes, which can result in head crashes or other data losses or errors.
In sum, there is a need for systems and methods that may adequately capture and/or process large area images in detail by, for example, utilization of one or more camera systems or arrays having image capturing/processing configurations that provide features such specified overlap characteristics, the ability to create detail photomaps, among others.